The present embodiments relate to emitting a sequence of high frequency pulses that may have different envelopes, in a magnetic resonance tomography system.
In order to generate an image with the aid of a magnetic resonance tomography method, defined high frequency pulses are to be emitted into the patient's body or into the part of the body that is to be investigated that is located in a precisely defined magnetic field. This stimulates the nuclear spins of the atoms in the object that is being investigated. The signals from the nuclear spin that have been emitted in sequence are captured and acquired as raw data, from which the desired magnetic resonance images may be generated. In general, a very wide range of sequences of high frequency pulses are emitted for various investigations, each individual high frequency pulse having a precisely defined duration, amplitude and shape in order to achieve a certain effect. The parameters used for this purpose are set in the form of a digital data stream that is mixed at a mixed frequency MF so that in total, the high frequency signal that has been modulated in the desired manner (e.g., the desired series of high frequency pulses) is generated.
High frequency signals vary depending on the load. The load may generate a complex reflection coefficient (e.g., part of the power output from the amplifier is reflected and phase-reversed). In a magnetic resonance tomography system, the load depends on each patient being investigated and on each part of the body investigated. Therefore, before the start of an investigation, pulses known as “adjustment pulses” are transmitted, with which the power output used for each investigation situation is determined in advance.
A movement on the part of the patient during the investigation may lead to a change in the load situation, however. The performance of some components during the duration of the investigation may vary due to temperature effects.
Carrying out a regulation of the amplitude and of the phase of the high frequency signal even during the investigation is therefore already known. This may involve taking into account both the signal from the amplifier to the load and the reflected signal.
In magnetic resonance tomography systems, the high frequency power amplifiers (e.g., RFPAs or radio frequency power amplifiers) incorporated therein, with which the high frequency transmission pulses are generated, are critical components. They may be used up to a power limit and may sustain damage permanently even when there is only a short-term overload.
Attempts may be made to avoid overload situations by using additional adjustment pulses that check the overload limit. Magnetic resonance tomography systems may also be fitted with overload detectors that detect a potentially damaging overload situation and trigger the aborting of the sequence. This provides, however, that the prescribed sequence of precisely defined high frequency pulses for an investigation is aborted. A new adjustment is then carried out using adjustment pulses. For the patient, the duration of the investigation is increased, and the equipment is taken up by an investigation for a longer period.